Most of the current Great Wall we see today was built in the Ming Dynasty (1368-1644) and is approximately 6000km long.
Although a useful deterrent against raids, at several points throughout its history the Great Wall failed to stop enemies, including in 1644 when the Manchu Qing marched through the gates of Shanhai Pass and replaced the most ardent of the wall-building dynasties, the Ming, as rulers of China.
Trump's wall would be little more than a monument to ignorance, stupidity, bigotry, and racism with his name on it....
Perhaps the headstone on his grave should be some kind of replica of his 'great wall' with the following inscription:
Humpty Trumpty has his great wall.
His biggest wish and his political fall.
All of his forces and all of his ken
couldn't make HIS America
be great again.